This paper outlines the design and prototyping of the ATLAS High Level Trigger (HLT) which is a combined effort of the Data Collection, HLT and PESA (Physics and Event Selection Architecture) subgroups within the ATLAS TDAQ collaboration. Two important issues, already outlined in the ATLAS HLT, DAQ and DCS Technical Proposal [1] will be highlighted: the treatment of the LVL2 Trigger and Event Filter as aspects of a general HLT with a view to easier migration of algorithms between the two levels; unification of the selective data collection for LVL2 and Event Building.